In aircraft towing vehicles without a towbar the nose wheel is supported and secured on a support surface designed as a platform, cradle, ramp or the like, provided in the fork-shaped, rearward open recess of the towing vehicle. In order to place the nose wheel on the support surface a take-up operation occurs, consisting mainly of two motion courses. In one variant, the nose wheel is gripped from behind by a gripping and traction device travelling back and forth on the towing vehicle in longitudinal direction and pulled onto the support surface which is basically stationarily mounted on the towing vehicle, such as known from German open specifications 33 18 077, 37 01 971 and 18 01 855. During this take-up operation the aircraft and the towing vehicle have to move relatively to each other, whereby due to the big difference in mass the aircraft stands still, preferably with set landing-gear brakes, while the towing vehicle moves in reverse during the take-up, until the support surface is positioned under the nose wheel In the other variant of the take-up operation both the aircraft and the towing vehicle are stationary relatively to each other and the support surface is moved backwards with respect to the towing vehicle and slipped under the nose wheel. A countersupport reaching behind the nose wheel takes care of the securing of the nose wheel with respect to the towing vehicle and prevents strain on the nose wheel gear due to horizontal forces acting on the nose wheel while the support surface is slipped under the latter. Towing vehicles of this kind are known from DE-OS 39 17 255 and EP 0 331 363 A1.
The invention relates to a towing vehicle of the kind first mentioned, wherein the nose wheel is gripped from the rear by a gripping device and pulled onto the support surface. A towing vehicle is known particularly from EP 0 276 779 A1. In this known towing vehicle the lifting cradle having the support surface is supported on the chassis so that it is swingable about a horizontal transverse axis and the traction arms carrying the gripping members are rigidly connected with the lifting cradle, so that when the lifting cradle is raised or lowered they will be moved up or down to the same extent, and that especially when the lifting cradle is raised they will move together with the same and the nose wheel. Since in this known towing vehicle the position of the gripping members does not change with respect to the lifting cradle during the raising and lowering operation, the gripping member presses against the same location on the nose wheel circumference when the lifting cradle is lowered, as well as when the lifting cradle is raised. If this location is above the axis of rotation of the nose wheel, the gripping member can exert a downwardly oriented holding force on the nose wheel, however there is the disadvantage that the downwardly oriented force component already exists when the nose wheel is supposed to be pulled onto the lowered lifting cradle. This is a drawback, since during the take-up it is desirable to have a force component which is again upwardly oriented, helping to overcome the difference in height between the runway and the lowered lifting cradle. From DE-OS 39 17 255 a towing vehicle is known which has an actively rearward slidable lifting cradle and therefore pertains to the above-mentioned second variant of the take-up operation, wherein traction arms bearing the gripping members reaching behind the nose wheel can be swung vertically independently from the lifting cradle, so that the gripping members can be applied against any desired location on the nose wheel circumference. However, additional hydraulic adjusting cylinders with corresponding control units are required, which increases the complexity and vulnerability of the entire pickup device. Further, both previously mentioned towing vehicles have the disadvantage that the lifting cradle and the traction arms are horizontally swingable o the towing vehicle about rigidly established axes, thereby not having any freedom of movement with respect to the longitudinal axis of the towing vehicle This means that the towing vehicle is under unilateral strain due to the load of the nose wheel positioned eccentrically with respect to the vehicle's longitudinal median plane, which can impair its travel capabilities. Such an eccentric strain exerted by the nose wheel can be the result of an inaccurately centered positioning of the nose wheel on the support surface on the one hand, and on the other hand of the fact that when the vehicle turns, due to normally longitudinally inclined longitudinal axis of the nose wheel, a steering angle of the nose wheel is combined with a weight displacement towards the inside of the curve. From the already mentioned EP 0 276 779 A1 an embodiment of the towing vehicle is known wherein the lifting cradle is raisably and lowerably guided on the chassis via four guide rods arranged in a parallelogram, whereby these guide rods allow for inclinations of the lifting cradle with respect to the longitudinal axis of the vehicle; however, transverse motions of the lifting cradle in order to center an eccentrically picked up nose wheel are not possible.